Enable default request timeout of 300 seconds (#870)

Sets a default request timeout of 300 seconds to fix Instaloader hanging indefinitely when used on an unstable internet connection without using --request-timeout, such as in #810.

docs/cli-options.rst

@@ -238,10 +238,13 @@ How to Download
 
 .. option:: --request-timeout N
 
-   Seconds to wait before timing out a connection request.
+   Seconds to wait before timing out a connection request. Defaults to 300.
 
    .. versionadded:: 4.3
 
+   .. versionchanged:: 4.6
+      Enabled this option by default with a timeout of 300 seconds.
+
 Miscellaneous Options
 ^^^^^^^^^^^^^^^^^^^^^